Various drug eluting devices have been described. Generally, a monolithic device will elute a drug with first order kinetics with a high initial release followed by a continually tapering release. When zero order steady release is desired, a more complex device is required.
Published U.S. Patent Application No. 2010/0034863 discloses the use of encapsulated active pharmaceutical ingredients in a monolithic device to achieve zero order release.
EP Patent 1,732,520 B1 discloses the use of a core sheath device with both the core and sheath being made of ethylene vinyl acetate. One disadvantage of using ethylene vinyl acetate as the core material is that only hydrophobic drugs are soluble in ethylene vinyl acetate and even then to relatively low degrees. In practice, this results in limiting the amount of drug that can be released on a daily basis from such rings. A further related disadvantage is that ethylene vinyl acetate is essentially non-water swellable, thus it does not allow for the exploitation of water soluble drugs.